1. Field of the Invention
The invention relates to the design of steam-heated, preferably metallic rolls which are put to use e.g. in paper-making machines, film/sheet drawing machines or similar machine equipment. For this purpose the rolls often need to be heated, for which liquid or gaseous heat transfer media find application. In one particular aspect of such rolls a plurality of drilled passages is provided located axially parallel near to the periphery, through which the heat transfer medium is guided. In this respect the invention involves the aspect in which steam is used as the heat transfer medium.
2. Description of the Prior Art
One such roll is described in DE-A-43 13 379 of the applicant. The embodiment of the roll is designed therein to provide for the steam directed through the peripheral drilled passages to condense therein at least in part, upon which the condensate, supported by centrifugal force, flows to the ends of the peripheral drilled passages at each end of the roll where, due to the vapor pressure or a negative pressure applied to a discharge conduit, it is forced through tubes or drilled passages to the axis of the roll from which it may be discharged from the roll through the drain conduit. One such tube or drilled passage is situated at each end of the peripheral drilled passages. In this arrangement the amount of condensate discharged can be controlled by a condensate control valve. Thus the heating performance of the roll can also be determined.
Condensate control valves of this kind have a proven record of success in comparable applications--such as e.g. in the steam heating of plate presses. However, when applied in conjunction with peripheral drilled rolls trouble in operation cannot be totally excluded due to the special features involved in operation. Slight irregularities in the amount of condensate materializing or, as a result of circulation, in dewatering the various discharge tubes or drilled passages may cause steam to be blown through the discharge tubes or drilled passages. This prompts the condensate control valve to shut off in advance. The further dewatering of the roll is restricted, making it impossible in the end.
It has been attempted to accelerate condensation of the blown steam through the discharge conduit, e.g. by means of non-insulated conduits or an additional condenser. This then causes the condensate control valve to reopen and condensate is again able to leave the roll. Experience has shown, however, that in this way too, totally consistent dewatering of all peripheral drilled passages is not always assured and that individual conduits may still remain filled with condensed water during operation. This results in lack of uniformity in heating the roll and thus in its thermal distortion as well as in imbalances in roll running.
Furthermore, attempts have been made to do away with the condensate control valve and, instead, to allow a certain flow of blow-through steam. This has the advantage that non-condensable gases gaining access to the roll and obstructing heat transfer are also swept from the roll. Systems for controlling the amount of steam blown through are known e.g. for drying cylinders in paper-making machines. A distinction is made here between differential pressure control systems and flow amount control systems.
The differential pressure control system comprises in the discharge conduit a throttle valve which maintains a specific differential pressure between the inflow conduit and the outflow conduit of the roll. If more steam than is required attempts to flow through the roll, the pressure differential of the conduits increases and the valve closes, and vice-versa. In flow amount control the amount of overblow steam in the outflow conduit is determined, e.g. at a restrictor, by measuring the pressure loss, and directly controlled via a correspondingly controlled throttle valve.
These two systems also fail to work reliably enough in the case of steam-heated rolls of the type incorporating peripheral drilled passages. Since either the common pressure difference is specified for all dewatering tubes of a peripherally drilled roll or the sum of the blow-through steam through all dewatering tubes is also measured in common, it may happen time and again that flow differences materialize in the individual dewatering tubes which lead, on the one hand, to intensified steam blow-through in individual tubes or a group thereof and, on the other, to flooding of individual or several peripheral drilled passages.